It is known in the art that certain types of hydrocracking catalysts comprising a Group VIII noble metal supported on an acidic Y zeolite base undergo gradual deactivation upon extended use in hydrocracking, which deactivation is attributable in part to coke deposition, and in part to migration and agglomeration of the noble metal component. Coke deactivation is readily reversible by conventional oxidative regeneration, but such regeneration has little if any effect in redispersing the agglomerated noble metal, and may in fact bring about further agglomeration thereof. Recently, as disclosed in U.S. Pat. Nos. 3,692,692, 3,899,441, and 3,943,051, it has been discovered that effective redispersal of agglomerated noble metal on such catalysts can be obtained by treatment with aqueous ammonia solutions. Following the aqueous ammonia treatment, a calcination step is required in order to convert the hydrated ammonium zeolite catalyst to a dehydrated, deammoniated, oxidized form. The temperature utilized in this calcination step form the gist of the present invention.
All previous experience in calcining the original, fresh ammonium form of the present catalysts indicated that temperatures between about 900.degree. and 1100.degree. F were required in order to achieve maximum activity. Based on this experience, each of the aforementioned patents strongly recommends that the ammonia-rejuvenated catalysts also be calcined at temperatures of 900.degree.-1100.degree. F. This premise came into question when it was discovered (as disclosed in my U.S. Pat. No. 4,002,575) that a substantial increase in activity could be obtained by partially rehydrating ammonia-rejuvenated catalysts which had been calcined at 930.degree. F. Based on this experience, it was surmised that a similar increase in activity might be obtained if an equivalent proportion of water were retained in the rejuvenated catalyst by simply calcining at lower temperature. In testing this premise, two rather distinct and surprising discoveries were made, which form the basis of the present invention.
Firstly, it was discovered that for hydrocracking in the presence of more than about 200 ppm by weight of ammoniacal nitrogen, the low-temperature calcined rejuvanated catalysts were at least equivalent in activity to partially rehydrated, high-temperature calcined catalysts of equivalent water content. Secondly, and more surprisingly, it was found that the former type catalyst (of this invention) was significantly more active than the latter for hydrocracking in the presence of less than about 200 ppm of ammoniacal nitrogen. Based on these discoveries, it would appear that the water retained by low temperature calcination following rejuvenation probably occupies more effective active sites in the anionic structure of the zeolite than does an equivalent proportion of water added by rehydration to a high temperature calcined rejuvenated catalyst. The foregoing explanation is however merely theoretical, and is not to be construed as limiting herein.